Context — The Mesa 3D Graphics Library latest documentation

# Context

A Gallium rendering context encapsulates the state which effects 3D rendering such as blend state, depth/stencil state, texture samplers, etc.

Note that resource/texture allocation is not per-context but per-screen.

## Methods

### CSO State

All Constant State Object (CSO) state is created, bound, and destroyed, with triplets of methods that all follow a specific naming scheme. For example, `create_blend_state`, `bind_blend_state`, and `destroy_blend_state`.

CSO objects handled by the context object:

- Blend: `*_blend_state`

- Sampler: Texture sampler states are bound separately for fragment, vertex, geometry and compute shaders with the `bind_sampler_states` function. The `start` and `num_samplers` parameters indicate a range of samplers to change. NOTE: at this time, start is always zero and the CSO module will always replace all samplers at once (no sub-ranges). This may change in the future.

- Rasterizer: `*_rasterizer_state`

- Depth, Stencil, & Alpha: `*_depth_stencil_alpha_state`

- Shader: These are create, bind and destroy methods for vertex, fragment and geometry shaders.

- Vertex Elements: `*_vertex_elements_state`

### Resource Binding State

This state describes how resources in various flavors (textures, buffers, surfaces) are bound to the driver.

- `set_constant_buffer` sets a constant buffer to be used for a given shader type. index is used to indicate which buffer to set (some APIs may allow multiple ones to be set, and binding a specific one later, though drivers are mostly restricted to the first one right now). If take_ownership is true, the buffer reference is passed to the driver, so that the driver doesn’t have to increment the reference count.

- `set_inlinable_constants` sets inlinable constants for constant buffer 0.

These are constants that the driver would like to inline in the IR of the current shader and recompile it. Drivers can determine which constants they prefer to inline in finalize_nir and store that information in shader_info::inlinable_uniform. When the state tracker or frontend uploads constants to a constant buffer, it can pass inlinable constants separately via this call.

Any `set_constant_buffer` call invalidates inlinable constants, so `set_inlinable_constants` must be called after it. Binding a shader also invalidates this state.

There is no `PIPE_CAP` for this. Drivers shouldn’t set the shader_info fields if they don’t implement `set_inlinable_constants`.

- `set_framebuffer_state`

- `set_vertex_buffers`

### Non-CSO State

These pieces of state are too small, variable, and/or trivial to have CSO objects. They all follow simple, one-method binding calls, e.g. `set_blend_color`.

- `set_stencil_ref` sets the stencil front and back reference values which are used as comparison values in stencil test.

- `set_blend_color`

- `set_sample_mask` sets the per-context multisample sample mask. Note that this takes effect even if multisampling is not explicitly enabled if the framebuffer surface(s) are multisampled. Also, this mask is AND-ed with the optional fragment shader sample mask output (when emitted).

- `set_sample_locations` sets the sample locations used for rasterization. ``get_sample_position`` still returns the default locations. When NULL, the default locations are used.

- `set_min_samples` sets the minimum number of samples that must be run.

- `set_clip_state`

- `set_polygon_stipple`

- `set_scissor_states` sets the bounds for the scissor test, which culls pixels before blending to render targets. If the Rasterizer does not have the scissor test enabled, then the scissor bounds never need to be set since they will not be used. Note that scissor xmin and ymin are inclusive, but xmax and ymax are exclusive. The inclusive ranges in x and y would be [xmin..xmax-1] and [ymin..ymax-1]. The number of scissors should be the same as the number of set viewports and can be up to PIPE_MAX_VIEWPORTS.

- `set_viewport_states`

- `set_window_rectangles` sets the window rectangles to be used for rendering, as defined by GL_EXT_window_rectangles. There are two modes - include and exclude, which define whether the supplied rectangles are to be used for including fragments or excluding them. All of the rectangles are ORed together, so in exclude mode, any fragment inside any rectangle would be culled, while in include mode, any fragment outside all rectangles would be culled. xmin/ymin are inclusive, while xmax/ymax are exclusive (same as scissor states above). Note that this only applies to draws, not clears or blits. (Blits have their own way to pass the requisite rectangles in.)

- `set_tess_state` configures the default tessellation parameters:

  - `default_outer_level` is the default value for the outer tessellation levels. This corresponds to GL’s `PATCH_DEFAULT_OUTER_LEVEL`.

  - `default_inner_level` is the default value for the inner tessellation levels. This corresponds to GL’s `PATCH_DEFAULT_INNER_LEVEL`.

- `set_patch_vertices` sets the number of vertices per input patch for tessellation.

- `set_debug_callback` sets the callback to be used for reporting various debug messages, eventually reported via GL_KHR_debug and similar mechanisms.

### Samplers

pipe_sampler_state objects control how textures are sampled (coordinate wrap modes, interpolation modes, etc). Samplers are only required for texture instructions for which nir_tex_instr_need_sampler returns true. Drivers must ignore samplers for other texture instructions. Frontends may or may not bind samplers when no texture instruction use them. Notably, frontends may not bind samplers for texture buffer objects, which are never accessed with samplers.

### Sampler Views

These are the means to bind textures to shader stages. To create one, specify its format, swizzle and LOD range in sampler view template.

If texture format is different than template format, it is said the texture is being cast to another format. Casting can be done only between compatible formats, that is formats that have matching component order and sizes.

Swizzle fields specify the way in which fetched texel components are placed in the result register. For example, `swizzle_r` specifies what is going to be placed in first component of result register.

The `first_level` and `last_level` fields of sampler view template specify the LOD range the texture is going to be constrained to. Note that these values are in addition to the respective min_lod, max_lod values in the pipe_sampler_state (that is if min_lod is 2.0, and first_level 3, the first mip level used for sampling from the resource is effectively the fifth).

The `first_layer` and `last_layer` fields specify the layer range the texture is going to be constrained to. Similar to the LOD range, this is added to the array index which is used for sampling.

- `set_sampler_views` binds an array of sampler views to a shader stage. Every binding point acquires a reference to a respective sampler view and releases a reference to the previous sampler view.

  Sampler views outside of `[start_slot, start_slot + num_views)` are unmodified. If `views` is NULL, the behavior is the same as if `views[n]` was NULL for the entire range, i.e. releasing the reference for all the sampler views in the specified range.

- `create_sampler_view` creates a new sampler view. `texture` is associated with the sampler view which results in sampler view holding a reference to the texture. Format specified in template must be compatible with texture format.

- `sampler_view_destroy` destroys a sampler view and releases its reference to associated texture.

### Hardware Atomic buffers

Buffers containing HW atomics are required to support the feature on some drivers.

Drivers that require this need to fill the `set_hw_atomic_buffers` method.

### Shader Resources

Shader resources are textures or buffers that may be read or written from a shader without an associated sampler. This means that they have no support for floating point coordinates, address wrap modes or filtering.

There are 2 types of shader resources: buffers and images.

Buffers are specified using the `set_shader_buffers` method.

Images are specified using the `set_shader_images` method. When binding images, the `level`, `first_layer` and `last_layer` pipe_image_view fields specify the mipmap level and the range of layers the image will be constrained to.

### Surfaces

These are the means to use resources as color render targets or depthstencil attachments. To create one, specify the mip level, the range of layers, and the bind flags (either PIPE_BIND_DEPTH_STENCIL or PIPE_BIND_RENDER_TARGET). Note that layer values are in addition to what is indicated by the geometry shader output variable XXX_FIXME (that is if first_layer is 3 and geometry shader indicates index 2, the 5th layer of the resource will be used). These first_layer and last_layer parameters will only be used for 1d array, 2d array, cube, and 3d textures otherwise they are 0.

- `create_surface` creates a new surface.

- `surface_destroy` destroys a surface and releases its reference to the associated resource.

### Stream output targets

Stream output, also known as transform feedback, allows writing the primitives produced by the vertex pipeline to buffers. This is done after the geometry shader or vertex shader if no geometry shader is present.

The stream output targets are views into buffer resources which can be bound as stream outputs and specify a memory range where it’s valid to write primitives. The pipe driver must implement memory protection such that any primitives written outside of the specified memory range are discarded.

Two stream output targets can use the same resource at the same time, but with a disjoint memory range.

Additionally, the stream output target internally maintains the offset into the buffer which is incremented every time something is written to it. The internal offset is equal to how much data has already been written. It can be stored in device memory and the CPU actually doesn’t have to query it.

The stream output target can be used in a draw command to provide the vertex count. The vertex count is derived from the internal offset discussed above.

- `create_stream_output_target` create a new target.

- `stream_output_target_destroy` destroys a target. Users of this should use pipe_so_target_reference instead.

- `set_stream_output_targets` binds stream output targets. The parameter offset is an array which specifies the internal offset of the buffer. The internal offset is, besides writing, used for reading the data during the draw_auto stage, i.e. it specifies how much data there is in the buffer for the purposes of the draw_auto stage. -1 means the buffer should be appended to, and everything else sets the internal offset.

- `stream_output_target_offset` Retrieve the internal stream offset from an streamout target. This is used to implement Vulkan pause/resume support which needs to pass the internal offset to the API.

NOTE: The currently-bound vertex or geometry shader must be compiled with the properly-filled-in structure pipe_stream_output_info describing which outputs should be written to buffers and how. The structure is part of pipe_shader_state.

### Clearing

Clear is one of the most difficult concepts to nail down to a single interface (due to both different requirements from APIs and also driver/HW specific differences).

`clear` initializes some or all of the surfaces currently bound to the framebuffer to particular RGBA, depth, or stencil values. Currently, this does not take into account color or stencil write masks (as used by GL), and always clears the whole surfaces (no scissoring as used by GL clear or explicit rectangles like d3d9 uses). It can, however, also clear only depth or stencil in a combined depth/stencil surface. If a surface includes several layers then all layers will be cleared.

`clear_render_target` clears a single color rendertarget with the specified color value. While it is only possible to clear one surface at a time (which can include several layers), this surface need not be bound to the framebuffer. If render_condition_enabled is false, any current rendering condition is ignored and the clear will be unconditional.

`clear_depth_stencil` clears a single depth, stencil or depth/stencil surface with the specified depth and stencil values (for combined depth/stencil buffers, it is also possible to only clear one or the other part). While it is only possible to clear one surface at a time (which can include several layers), this surface need not be bound to the framebuffer. If render_condition_enabled is false, any current rendering condition is ignored and the clear will be unconditional.

`clear_texture` clears a non-PIPE_BUFFER resource’s specified level and bounding box with a clear value provided in that resource’s native format.

`clear_buffer` clears a PIPE_BUFFER resource with the specified clear value (which may be multiple bytes in length). Logically this is a memset with a multi-byte element value starting at offset bytes from resource start, going for size bytes. It is guaranteed that size % clear_value_size == 0.

### Evaluating Depth Buffers

`evaluate_depth_buffer` is a hint to decompress the current depth buffer assuming the current sample locations to avoid problems that could arise when using programmable sample locations.

If a depth buffer is rendered with different sample location state than what is current at the time of reading the depth buffer, the values may differ because depth buffer compression can depend the sample locations.

### Uploading

For simple single-use uploads, use `pipe_context::stream_uploader` or `pipe_context::const_uploader`. The latter should be used for uploading constants, while the former should be used for uploading everything else. PIPE_USAGE_STREAM is implied in both cases, so don’t use the uploaders for static allocations.

Usage:

Call u_upload_alloc or u_upload_data as many times as you want. After you are done, call u_upload_unmap. If the driver doesn’t support persistent mappings, u_upload_unmap makes sure the previously mapped memory is unmapped.

Gotchas: - Always fill the memory immediately after u_upload_alloc. Any following call to u_upload_alloc and u_upload_data can unmap memory returned by previous u_upload_alloc. - Don’t interleave calls using stream_uploader and const_uploader. If you use one of them, do the upload, unmap, and only then can you use the other one.

### Drawing

`draw_vbo` draws a specified primitive. The primitive mode and other properties are described by `pipe_draw_info`.

The `mode`, `start`, and `count` fields of `pipe_draw_info` specify the the mode of the primitive and the vertices to be fetched, in the range between `start` to `start``+``count`-1, inclusive.

Every instance with instanceID in the range between `start_instance` and `start_instance``+``instance_count`-1, inclusive, will be drawn.

If `index_size` != 0, all vertex indices will be looked up from the index buffer.

In indexed draw, `min_index` and `max_index` respectively provide a lower and upper bound of the indices contained in the index buffer inside the range between `start` to `start``+``count`-1. This allows the driver to determine which subset of vertices will be referenced during the draw call without having to scan the index buffer. Providing a over-estimation of the the true bounds, for example, a `min_index` and `max_index` of 0 and 0xffffffff respectively, must give exactly the same rendering, albeit with less performance due to unreferenced vertex buffers being unnecessarily DMA’ed or processed. Providing a underestimation of the true bounds will result in undefined behavior, but should not result in program or system failure.

In case of non-indexed draw, `min_index` should be set to `start` and `max_index` should be set to `start``+``count`-1.

`index_bias` is a value added to every vertex index after lookup and before fetching vertex attributes.

When drawing indexed primitives, the primitive restart index can be used to draw disjoint primitive strips. For example, several separate line strips can be drawn by designating a special index value as the restart index. The `primitive_restart` flag enables/disables this feature. The `restart_index` field specifies the restart index value.

When primitive restart is in use, array indexes are compared to the restart index before adding the index_bias offset.

If a given vertex element has `instance_divisor` set to 0, it is said it contains per-vertex data and effective vertex attribute address needs to be recalculated for every index.

> attribAddr = `stride` * index + `src_offset`

If a given vertex element has `instance_divisor` set to non-zero, it is said it contains per-instance data and effective vertex attribute address needs to recalculated for every `instance_divisor`-th instance.

> attribAddr = `stride` * instanceID / `instance_divisor` + `src_offset`

In the above formulas, `src_offset` is taken from the given vertex element and `stride` is taken from a vertex buffer associated with the given vertex element.

The calculated attribAddr is used as an offset into the vertex buffer to fetch the attribute data.

The value of `instanceID` can be read in a vertex shader through a system value register declared with INSTANCEID semantic name.

### Queries

Queries gather some statistic from the 3D pipeline over one or more draws. Queries may be nested, though not all gallium frontends exercise this.

Queries can be created with `create_query` and deleted with `destroy_query`. To start a query, use `begin_query`, and when finished, use `end_query` to end the query.

`create_query` takes a query type (`PIPE_QUERY_*`), as well as an index, which is the vertex stream for `PIPE_QUERY_PRIMITIVES_GENERATED` and `PIPE_QUERY_PRIMITIVES_EMITTED`, and allocates a query structure.

`begin_query` will clear/reset previous query results.

`get_query_result` is used to retrieve the results of a query. If the `wait` parameter is TRUE, then the `get_query_result` call will block until the results of the query are ready (and TRUE will be returned). Otherwise, if the `wait` parameter is FALSE, the call will not block and the return value will be TRUE if the query has completed or FALSE otherwise.

`get_query_result_resource` is used to store the result of a query into a resource without synchronizing with the CPU. This write will optionally wait for the query to complete, and will optionally write whether the value is available instead of the value itself.

`set_active_query_state` Set whether all current non-driver queries except TIME_ELAPSED are active or paused.

The interface currently includes the following types of queries:

`PIPE_QUERY_OCCLUSION_COUNTER` counts the number of fragments which are written to the framebuffer without being culled by Depth, Stencil, & Alpha testing or shader KILL instructions. The result is an unsigned 64-bit integer. This query can be used with `render_condition`.

In cases where a boolean result of an occlusion query is enough, `PIPE_QUERY_OCCLUSION_PREDICATE` should be used. It is just like `PIPE_QUERY_OCCLUSION_COUNTER` except that the result is a boolean value of FALSE for cases where COUNTER would result in 0 and TRUE for all other cases. This query can be used with `render_condition`.

In cases where a conservative approximation of an occlusion query is enough, `PIPE_QUERY_OCCLUSION_PREDICATE_CONSERVATIVE` should be used. It behaves like `PIPE_QUERY_OCCLUSION_PREDICATE`, except that it may return TRUE in additional, implementation-dependent cases. This query can be used with `render_condition`.

`PIPE_QUERY_TIME_ELAPSED` returns the amount of time, in nanoseconds, the context takes to perform operations. The result is an unsigned 64-bit integer.

`PIPE_QUERY_TIMESTAMP` returns a device/driver internal timestamp, scaled to nanoseconds, recorded after all commands issued prior to `end_query` have been processed. This query does not require a call to `begin_query`. The result is an unsigned 64-bit integer.

`PIPE_QUERY_TIMESTAMP_RAW` returns a device/driver timestamp, recorded after all commands issued prior to `end_query` have been processed. This query does not require a call to `begin_query`. The result is an unsigned 64-bit integer. `pipe_screen::convert_timestamp()` should be implemented, to covert the raw timestamp to nanoseconds, if this query type is supported. `PIPE_QUERY_TIMESTAMP_DISJOINT` can be used to check the internal timer resolution and whether the timestamp counter has become unreliable due to things like throttling etc. - only if this is FALSE a timestamp query (within the timestamp_disjoint query) should be trusted. The result is a 64-bit integer specifying the timer resolution in Hz, followed by a boolean value indicating whether the timestamp counter is discontinuous or disjoint.

`PIPE_QUERY_PRIMITIVES_GENERATED` returns a 64-bit integer indicating the number of primitives processed by the pipeline (regardless of whether stream output is active or not).

`PIPE_QUERY_PRIMITIVES_EMITTED` returns a 64-bit integer indicating the number of primitives written to stream output buffers.

`PIPE_QUERY_SO_STATISTICS` returns 2 64-bit integers corresponding to the result of `PIPE_QUERY_PRIMITIVES_EMITTED` and the number of primitives that would have been written to stream output buffers if they had infinite space available (primitives_storage_needed), in this order. XXX the 2nd value is equivalent to `PIPE_QUERY_PRIMITIVES_GENERATED` but it is unclear if it should be increased if stream output is not active.

`PIPE_QUERY_SO_OVERFLOW_PREDICATE` returns a boolean value indicating whether a selected stream output target has overflowed as a result of the commands issued between `begin_query` and `end_query`. This query can be used with `render_condition`. The output stream is selected by the stream number passed to `create_query`.

`PIPE_QUERY_SO_OVERFLOW_ANY_PREDICATE` returns a boolean value indicating whether any stream output target has overflowed as a result of the commands issued between `begin_query` and `end_query`. This query can be used with `render_condition`, and its result is the logical OR of multiple `PIPE_QUERY_SO_OVERFLOW_PREDICATE` queries, one for each stream output target.

`PIPE_QUERY_GPU_FINISHED` returns a boolean value indicating whether all commands issued before `end_query` have completed. However, this does not imply serialization. This query does not require a call to `begin_query`.

`PIPE_QUERY_PIPELINE_STATISTICS` returns an array of the following 64-bit integers: Number of vertices read from vertex buffers. Number of primitives read from vertex buffers. Number of vertex shader threads launched. Number of geometry shader threads launched. Number of primitives generated by geometry shaders. Number of primitives forwarded to the rasterizer. Number of primitives rasterized. Number of fragment shader threads launched. Number of tessellation control shader threads launched. Number of tessellation evaluation shader threads launched. If a shader type is not supported by the device/driver, the corresponding values should be set to 0.

`PIPE_QUERY_PIPELINE_STATISTICS_SINGLE` returns a single counter from the `PIPE_QUERY_PIPELINE_STATISTICS` group. The specific counter must be selected when calling `create_query` by passing one of the `PIPE_STAT_QUERY` enums as the query’s `index`.

Gallium does not guarantee the availability of any query types; one must always check the capabilities of the Screen first.

### Conditional Rendering

A drawing command can be skipped depending on the outcome of a query (typically an occlusion query, or streamout overflow predicate). The `render_condition` function specifies the query which should be checked prior to rendering anything. Functions always honoring render_condition include (and are limited to) draw_vbo and clear. The blit, clear_render_target and clear_depth_stencil functions (but not resource_copy_region, which seems inconsistent) can also optionally honor the current render condition.

If `render_condition` is called with `query` = NULL, conditional rendering is disabled and drawing takes place normally.

If `render_condition` is called with a non-null `query` subsequent drawing commands will be predicated on the outcome of the query. Commands will be skipped if `condition` is equal to the predicate result (for non-boolean queries such as OCCLUSION_QUERY, zero counts as FALSE, non-zero as TRUE).

If `mode` is PIPE_RENDER_COND_WAIT the driver will wait for the query to complete before deciding whether to render.

If `mode` is PIPE_RENDER_COND_NO_WAIT and the query has not yet completed, the drawing command will be executed normally. If the query has completed, drawing will be predicated on the outcome of the query.

If `mode` is PIPE_RENDER_COND_BY_REGION_WAIT or PIPE_RENDER_COND_BY_REGION_NO_WAIT rendering will be predicated as above for the non-REGION modes but in the case that an occlusion query returns a non-zero result, regions which were occluded may be omitted by subsequent drawing commands. This can result in better performance with some GPUs. Normally, if the occlusion query returned a non-zero result subsequent drawing happens normally so fragments may be generated, shaded and processed even where they’re known to be obscured.

The ‘’render_condition_mem’’ function specifies the drawing is dependent on a value in memory. A buffer resource and offset denote which 32-bit value to use for the query. This is used for Vulkan API.

### Flushing

`flush`

PIPE_FLUSH_END_OF_FRAME: Whether the flush marks the end of frame.

PIPE_FLUSH_DEFERRED: It is not required to flush right away, but it is required to return a valid fence. If fence_finish is called with the returned fence and the context is still unflushed, and the ctx parameter of fence_finish is equal to the context where the fence was created, fence_finish will flush the context.

PIPE_FLUSH_ASYNC: The flush is allowed to be asynchronous. Unlike `PIPE_FLUSH_DEFERRED`, the driver must still ensure that the returned fence will finish in finite time. However, subsequent operations in other contexts of the same screen are no longer guaranteed to happen after the flush. Drivers which use this flag must implement pipe_context::fence_server_sync.

PIPE_FLUSH_HINT_FINISH: Hints to the driver that the caller will immediately wait for the returned fence.

Additional flags may be set together with `PIPE_FLUSH_DEFERRED` for even finer-grained fences. Note that as a general rule, GPU caches may not have been flushed yet when these fences are signaled. Drivers are free to ignore these flags and create normal fences instead. At most one of the following flags can be specified:

PIPE_FLUSH_TOP_OF_PIPE: The fence should be signaled as soon as the next command is ready to start executing at the top of the pipeline, before any of its data is actually read (including indirect draw parameters).

PIPE_FLUSH_BOTTOM_OF_PIPE: The fence should be signaled as soon as the previous command has finished executing on the GPU entirely (but data written by the command may still be in caches and inaccessible to the CPU).

`flush_resource`

Flush the resource cache, so that the resource can be used by an external client. Possible usage: - flushing a resource before presenting it on the screen - flushing a resource if some other process or device wants to use it This shouldn’t be used to flush caches if the resource is only managed by a single pipe_screen and is not shared with another process. (i.e. you shouldn’t use it to flush caches explicitly if you want to e.g. use the resource for texturing)

### Fences

`pipe_fence_handle`, and related methods, are used to synchronize execution between multiple parties. Examples include CPU <-> GPU synchronization, renderer <-> windowing system, multiple external APIs, etc.

A `pipe_fence_handle` can either be ‘one time use’ or ‘re-usable’. A ‘one time use’ fence behaves like a traditional GPU fence. Once it reaches the signaled state it is forever considered to be signaled.

Once a re-usable `pipe_fence_handle` becomes signaled, it can be reset back into an unsignaled state. The `pipe_fence_handle` will be reset to the unsignaled state by performing a wait operation on said object, i.e. `fence_server_sync`. As a corollary to this behavior, a re-usable `pipe_fence_handle` can only have one waiter.

This behavior is useful in producer <-> consumer chains. It helps avoid unnecessarily sharing a new `pipe_fence_handle` each time a new frame is ready. Instead, the fences are exchanged once ahead of time, and access is synchronized through GPU signaling instead of direct producer <-> consumer communication.

`fence_server_sync` inserts a wait command into the GPU’s command stream.

`fence_server_signal` inserts a signal command into the GPU’s command stream.

There are no guarantees that the wait/signal commands will be flushed when calling `fence_server_sync` or `fence_server_signal`. An explicit call to `flush` is required to make sure the commands are emitted to the GPU.

The Gallium implementation may implicitly `flush` the command stream during a `fence_server_sync` or `fence_server_signal` call if necessary.

Gallium frontends must ensure that `fence_server_signal` returns before calling `fence_server_sync` on the same `pipe_fence_handle` even if those calls are made on a different `pipe_context`.

### Resource Busy Queries

`is_resource_referenced`

### Blitting

These methods emulate classic blitter controls.

These methods operate directly on `pipe_resource` objects, and stand apart from any 3D state in the context. Each method is assumed to have an implicit memory barrier around itself. They do not need any explicit `memory_barrier`. Blitting functionality may be moved to a separate abstraction at some point in the future.

`resource_copy_region` blits a region of a resource to a region of another resource, provided that both resources have the same format, or compatible formats, i.e., formats for which copying the bytes from the source resource unmodified to the destination resource will achieve the same effect of a textured quad blitter.. The source and destination may be the same resource, but overlapping blits are not permitted. This can be considered the equivalent of a CPU memcpy.

`blit` blits a region of a resource to a region of another resource, including scaling, format conversion, and up-/downsampling, as well as a destination clip rectangle (scissors) and window rectangles. It can also optionally honor the current render condition (but either way the blit itself never contributes anything to queries currently gathering data). As opposed to manually drawing a textured quad, this lets the pipe driver choose the optimal method for blitting (like using a special 2D engine), and usually offers, for example, accelerated stencil-only copies even where pipe_caps.shader_stencil_export is not available.

### Transfers

These methods are used to get data to/from a resource.

`transfer_map` creates a memory mapping and the transfer object associated with it. The returned pointer points to the start of the mapped range according to the box region, not the beginning of the resource. If transfer_map fails, the returned pointer to the buffer memory is NULL, and the pointer to the transfer object remains unchanged (i.e. it can be non-NULL).

When mapping an MSAA surface, the samples are implicitly resolved to single-sampled for reads (returning the first sample for depth/stencil/integer, averaged for others). See u_transfer_helper’s U_TRANSFER_HELPER_MSAA_MAP for a way to get that behavior using a resolve blit.

`transfer_unmap` remove the memory mapping for and destroy the transfer object. The pointer into the resource should be considered invalid and discarded.

`texture_subdata` and `buffer_subdata` perform a simplified transfer for simple writes. Basically transfer_map, data write, and transfer_unmap all in one.

The box parameter to some of these functions defines a 1D, 2D or 3D region of pixels. This is self-explanatory for 1D, 2D and 3D texture targets.

For PIPE_TEXTURE_1D_ARRAY and PIPE_TEXTURE_2D_ARRAY, the box::z and box::depth fields refer to the array dimension of the texture.

For PIPE_TEXTURE_CUBE, the box:z and box::depth fields refer to the faces of the cube map (z + depth <= 6).

For PIPE_TEXTURE_CUBE_ARRAY, the box:z and box::depth fields refer to both the face and array dimension of the texture (face = z % 6, array = z / 6).

#### transfer_flush_region

If a transfer was created with `FLUSH_EXPLICIT`, it will not automatically be flushed on write or unmap. Flushes must be requested with `transfer_flush_region`. Flush ranges are relative to the mapped range, not the beginning of the resource.

#### texture_barrier

This function flushes all pending writes to the currently-set surfaces and invalidates all read caches of the currently-set samplers. This can be used for both regular textures as well as for framebuffers read via FBFETCH.

#### memory_barrier

This function flushes caches according to which of the PIPE_BARRIER_* flags are set.

#### resource_commit

This function changes the commit state of a part of a sparse resource. Sparse resources are created by setting the `PIPE_RESOURCE_FLAG_SPARSE` flag when calling `resource_create`. Initially, sparse resources only reserve a virtual memory region that is not backed by memory (i.e., it is uncommitted). The `resource_commit` function can be called to commit or uncommit parts (or all) of a resource. The driver manages the underlying backing memory.

The contents of newly committed memory regions are undefined. Calling this function to commit an already committed memory region is allowed and leaves its content unchanged. Similarly, calling this function to uncommit an already uncommitted memory region is allowed.

For buffers, the given box must be aligned to multiples of `pipe_caps.sparse_buffer_page_size`. As an exception to this rule, if the size of the buffer is not a multiple of the page size, changing the commit state of the last (partial) page requires a box that ends at the end of the buffer (i.e., box->x + box->width == buffer->width0).

### PIPE_MAP

These flags control the behavior of a transfer object.

`PIPE_MAP_READ`

  Resource contents read back (or accessed directly) at transfer create time.

`PIPE_MAP_WRITE`

  Resource contents will be written back at transfer_unmap time (or modified as a result of being accessed directly).

`PIPE_MAP_DIRECTLY`

  a transfer should directly map the resource. May return NULL if not supported.

`PIPE_MAP_DISCARD_RANGE`

  The memory within the mapped region is discarded. Cannot be used with `PIPE_MAP_READ`.

`PIPE_MAP_DISCARD_WHOLE_RESOURCE`

  Discards all memory backing the resource. It should not be used with `PIPE_MAP_READ`.

`PIPE_MAP_DONTBLOCK`

  Fail if the resource cannot be mapped immediately.

`PIPE_MAP_UNSYNCHRONIZED`

  Do not synchronize pending operations on the resource when mapping. The interaction of any writes to the map and any operations pending on the resource are undefined. Cannot be used with `PIPE_MAP_READ`.

`PIPE_MAP_FLUSH_EXPLICIT`

  Written ranges will be notified later with transfer_flush_region. Cannot be used with `PIPE_MAP_READ`.

`PIPE_MAP_PERSISTENT`

  Allows the resource to be used for rendering while mapped. PIPE_RESOURCE_FLAG_MAP_PERSISTENT must be set when creating the resource. If COHERENT is not set, memory_barrier(PIPE_BARRIER_MAPPED_BUFFER) must be called to ensure the device can see what the CPU has written.

`PIPE_MAP_COHERENT`

  If PERSISTENT is set, this ensures any writes done by the device are immediately visible to the CPU and vice versa. PIPE_RESOURCE_FLAG_MAP_COHERENT must be set when creating the resource.

### Compute kernel execution

A compute program can be defined, bound or destroyed using `create_compute_state`, `bind_compute_state` or `destroy_compute_state` respectively.

Any of the subroutines contained within the compute program can be executed on the device using the `launch_grid` method. This method will execute as many instances of the program as elements in the specified N-dimensional grid, hopefully in parallel.

The compute program has access to four special resources:

- `GLOBAL` represents a memory space shared among all the threads running on the device. An arbitrary buffer created with the `PIPE_BIND_GLOBAL` flag can be mapped into it using the `set_global_binding` method.

- `LOCAL` represents a memory space shared among all the threads running in the same working group. The initial contents of this resource are undefined.

- `PRIVATE` represents a memory space local to a single thread. The initial contents of this resource are undefined.

- `INPUT` represents a read-only memory space that can be initialized at `launch_grid` time.

These resources use a byte-based addressing scheme, and they can be accessed from the compute program by means of the LOAD/STORE TGSI opcodes.

In addition, normal texture sampling is allowed from the compute program: `bind_sampler_states` may be used to set up texture samplers for the compute stage and `set_sampler_views` may be used to bind a number of sampler views to it.

### Compute kernel queries

#### get_compute_state_info

This function allows frontends to query kernel information defined inside `pipe_compute_state_object_info`.

#### get_compute_state_subgroup_size

This function returns the choosen subgroup size when `launch_grid` is called with the given block size. This doesn’t need to be implemented when only one size is reported through `pipe_compute_caps.subgroup_sizes` or `pipe_compute_state_object_info::simd_sizes`.

### Mipmap generation

If pipe_caps.generate_mipmap is true, `generate_mipmap` can be used to generate mipmaps for the specified texture resource. It replaces texel image levels base_level+1 through last_level for layers range from first_layer through last_layer. It returns TRUE if mipmap generation succeeds, otherwise it returns FALSE. Mipmap generation may fail when it is not supported for particular texture types or formats.

### Device resets

Gallium frontends can query or request notifications of when the GPU is reset for whatever reason (application error, driver error). When a GPU reset happens, the context becomes unusable and all related state should be considered lost and undefined. Despite that, context notifications are single-shot, i.e. subsequent calls to `get_device_reset_status` will return PIPE_NO_RESET.

- `get_device_reset_status` queries whether a device reset has happened since the last call or since the last notification by callback.

- `set_device_reset_callback` sets a callback which will be called when a device reset is detected. The callback is only called synchronously.

### Bindless

If pipe_caps.bindless_texture is TRUE, the following `pipe_context` functions are used to create/delete bindless handles, and to make them resident in the current context when they are going to be used by shaders.

- `create_texture_handle` creates a 64-bit unsigned integer texture handle that is going to be directly used in shaders.

- `delete_texture_handle` deletes a 64-bit unsigned integer texture handle.

- `make_texture_handle_resident` makes a 64-bit unsigned texture handle resident in the current context to be accessible by shaders for texture mapping.

- `create_image_handle` creates a 64-bit unsigned integer image handle that is going to be directly used in shaders.

- `delete_image_handle` deletes a 64-bit unsigned integer image handle.

- `make_image_handle_resident` makes a 64-bit unsigned integer image handle resident in the current context to be accessible by shaders for image loads, stores and atomic operations.

## Using several contexts

Several contexts from the same screen can be used at the same time. Objects created on one context cannot be used in another context, but the objects created by the screen methods can be used by all contexts.

### Transfers

A transfer on one context is not expected to synchronize properly with rendering on other contexts, thus only areas not yet used for rendering should be locked.

A flush is required after transfer_unmap to expect other contexts to see the uploaded data, unless:

- Using persistent mapping. Associated with coherent mapping, unmapping the resource is also not required to use it in other contexts. Without coherent mapping, memory_barrier(PIPE_BARRIER_MAPPED_BUFFER) should be called on the context that has mapped the resource. No flush is required.

- Mapping the resource with PIPE_MAP_DIRECTLY.